It is known in the art to employ within a laser resonance cavity, e.g., defined as a laser chamber between a partially reflective output coupler and a fully reflective mirror forming the cavity, e.g., in a single chamber laser oscillator or an oscillator portion of a two chambered laser system having a oscillator portion feeding a seed beam into an amplifying portion, e.g., a power amplifier in a master oscillator power amplifier (“MOPA”) configuration, a line narrowing module. The line narrowing module is positioned and adapted to select a desired center wavelength around a narrow band of wavelengths, with the bandwidth of the narrow band also being carefully selected ordinarily to be of as narrow a bandwidth as possible, e.g., for lithography uses where chromatic aberrations in the lenses of a scanning lithography photo-resist exposure apparatus can be critical, but also to, e.g., be within some range of bandwidths, i.e., neither to large not too small, also, e.g., for photo-lithography reasons, e.g., for optimizing and enabling modern optical proximity correction techniques commonly used in preparing masks (reticles). For such reasons control of bandwidth in more than just a “not-to-exceed” mode is required, i.e., control is required within a narrow range of “not-to-exceed” and “not-to-go-below” specified values of bandwidth, and including with these requirements stability pulse to pulse.
It is also well known that such line narrowing modules may employ a variety of center wavelength selection optical elements, usually of the dispersive variety, which can reflect back into the optical path of, e.g., the laser oscillating resonance chamber light of the selected center wavelength and of a narrowed bandwidth, depending on a number of physical parameters of the line narrowing module and optical parameters and performance capabilities of the wavelength selective optical element, e.g., a dispersive optical element used.
In one commonly used line narrowing module of the type just described a reflective grating, e.g., an eschelle grating having a selected blaze angle and mounted in a Littrow configuration in the line narrowing module may be tuned to reflect back into the optical path of the laser oscillating resonance cavity light of a particular center wavelength, in part determined by the angle of incidence of the light in a laser light pulse beam pulse in the line narrowing module upon the dispersive optical element, e.g., the grating. Applicants' assignee's above noted patents show examples of such line narrowing modules.
It is also known in the art, as also exemplified in the above noted patents of applicants' assignee that one manner of controlling the angle of incidence of the laser light pulse beam upon the grating may be to employ a maximally reflective mirror for the desired center wavelength, e.g., 193 nm (KrF excimer lasers) or 248 nm (ArF excimer lasers), so-called by applicants' assignee and RMAX to reflect the laser light pulse beam passing through the line narrowing module upon the dispersive optical surface, e.g., the face of the grating.
Similarly, also as exemplified by the above referenced applicants' assignee's patents, it is well known that the laser light pulse beam may be expanded in the line narrowing module before being incident on wither the RMAX, or equivalent, and the grating, or equivalent, for several reasons. Beam expansion may be employed for reasons of protecting the optical elements, including down stream expansion optics, the RMAX and/or the grating from high levels of fluence energy, even more critical as wavelength decreases below about 300 nm, e.g., at 248 nm and more critically at 193 nm, and more so still at 157 nm (molecular fluorine excimer lasers). Beam Expansion may also be employed to magnify the beam and thereby reduce the impact of beam dispersion characteristic in, e.g., fluorine gas discharge excimer/molecular fluorine lasers and improve the center wavelength selection of the grating and thus also the narrowing of the bandwidth, so-called line narrowing the laser output.
It is also well known in the art that, e.g., it may be desirable to rapidly control the selection of the angle of incidence of the laser light pulse beam on the grating, e.g., to control center wavelength based on feedback control on a pulse-to-pulse basis and/or to engineer an integrated spectrum comprising the net effect of the wavelength spectra for pulses in the output laser light pulse beam output by the laser system for purposes of controlling such things as a broadened depth of focus in, e.g., a lithography scanning apparatus. Existing angle of incidence selection mechanisms for such tuning, e.g., the RMAX and equivalents have some limits in this area, e.g., due to mechanical resonances and the bulk of the RMAX required to be moved at very rapid periodic rates, e.g., 2-4 kHz and above, and the limitations of moving such optical elements with different (though related) rotating mechanisms for both coarse and fine adjustment of the angle of incidence during operation of the line narrowing module.
U.S. Pat. No. 6,760,358, issued to Zimmerman, et al. on Jul. 6, 2004, entitled LINE-NARROWING OPTICS MODULE HAVING IMPROVED MECHANICAL PERFORMANCE, the disclosure of which is hereby incorporated by reference, discloses:                An apparatus for adjusting an orientation of an optical component mounted within a laser resonator with suppressed hysteresis includes an electromechanical device, a drive element, and a mechano-optical device coupled to the mounted optical component. The drive element is configured to contact and apply a force to the mechano-optical device in such a way as to adjust the orientation of the mechano-optical device, and thereby that of the optical component, to a known orientation within the laser resonator. The optical component is mounted such that stresses applied by the mount to the optical component are homogeneous and substantially thermally-independent.        
It is known to move the dispersive optical element, e.g., a grating or an etalon for center wavelength control, as evidenced by U.S. patent Nos. or to move a beam expansion optical element with a fixed grating, in lieu of, e.g., using a rotatably positionable mirror, such as the RMAX or equivalents.
A need in the art exists, however, for a more effective line narrowing module which can, e.g., maintain or improve the center wavelength selection and control, e.g., wavelength stability pulse-to-pulse, using, e.g., a grating center wavelength selection element and means, including or in lieu of the RMAX for relatively simultaneous coarse and fine control of the angle of incidence of the laser light pulse beam pulses on the dispersive optical element, e.g., the grating. Applicants, according to aspects of embodiments of the present invention, have provided such improvements and modifications.
Diffraction grating have been known to fail, e.g., in ArF excimer laser LNM's. Applicants suspect that this failure is due at least in part to photo ionization of the aluminum underlayer on the grating, and subsequent oxidation reaction with O2. It is also clear that oxygen diffuses through defects and potentially the bulk MgF2 coating on the grating face, which may in some cases be alleviated with a coating(s) on the grating face. In investigating how to extend, e.g., ArF grating lifetimes, applicants have noted that, in general, grating lifetime is strongly influenced by oxygen levels in the LNM. The lower the oxygen content in ppm, the better. In addition, the ArF grating failure mode appears faster than the KrF grating failure mode, which applicants suspect is due to the fact that at around 193 nm an ArF photon is able to ionize the imbedded Al layer, thereby actually activating it for oxygen attack. While applicants are not sure of this fact, it appears that at around the 248 nm KrF photon is not energetic enough to activate the Al and help it corrode in the presence of oxygen. Grating degradation is still oxygen content related, but in the case of KrF photons appears to be limited to oxygen transport through the MgF2 layer (or reaction with imbedded oxygen).
Applicants propose a solution to grating lifetime degradation due to oxygen content in the LNM and also more specifically to accelerated degradation under the influence of higher energy photons.